The present invention relates to an anti-lock braking system (hereinafter referred to as an ABS) which comprises means for estimating, from a small number of parameters, the gradient of friction torque that wheels of a vehicle receive from the road surface(hereinafter referred to as a friction torque gradient) with respect to a slip speed, and which controls a braking force acting on wheels on the basis of the friction torque gradient estimated by the friction torque gradient estimating means.
More specifically, the friction torque gradient estimating means may employ several types of estimating methods; e.g., a method of estimating the friction torque gradient from only time-series data concerning a wheel speed; a method of estimating the friction torque gradient from time-series data concerning wheel deceleration as well as time-series data concerning the braking torque or physical quantities associated with the braking torque; or a method of estimating the friction torque gradient from micro-gains which are obtained when brake pressure is excited in a very small amount at the resonance frequency of a vibration system comprised of a vehicle body, wheels, and a road surface and which represent the characteristics of the vibration system.
Further, there is also a method available for determining, from the thus-estimated friction torque gradient, the limit of the characteristics of friction torque developed between the wheels and the road surface. As a result of the limit determination means determining the limit by this determination method, the starting point for anti-lock brake control and other various control operations can be accurately obtained.
The ABS control method based on the estimated gradient of friction torque is divided into two methods: namely, a servo control method by which the quantity of behavior of the wheel, or a physical quantity associated with the motion of the wheel, is controlled so as to follow a target value; and a method by which an estimated gradient of friction torque is controlled so as to match a reference value.
In the servo control method when the limit of the friction torque characteristics is determined by the limit determination means, arithmetic operations are executed in order to correct a preset target quantity of behavior of the wheel. As a result, stable anti-lock braking operation can be ensured on a road surface where the gradient of friction torque drastically changes in the vicinity of the peak friction coefficient (.mu.).
According to anti-lock brake control in which the gradient of friction torque is controlled so as to match a reference value, the accuracy of estimation is significantly improved by estimating the friction torque gradient through use of a small number of parameters, whereby more accurate anti-lock brake control is implemented. More particularly, in a case where a micro-gain (which is one type of representation of the friction torque gradient) is controlled so as to follow a reference value, the brake pressure of only the reference wheel is excited in a very small amount. In order to prevent interference between wheels due to the small vibration of the brake pressure, the other wheels are subject to at least either wheel-speed follow-up servo control or braking-force follow-up servo control.